Alyssa Hayden has won the Darling Range by-election, with the WA Liberal Party regaining the seat for the first time since the party was in government.

The party claimed the seat with seven per cent margin over WA Labor’s candidate Tania Lawrence as she and premier Mark McGowan conceded defeat.

“Everyone in the seat of Darling Range, thank you for giving me your trust,” Ms Hayden said to a crowd of cheering supporters on Saturday night at the party’s by-election base in Byford.

“I will deliver back on that. You have my word.”

Ms Hayden was ecstatic about her election victory but said her win “was a team effort”.

“I didn’t do this alone, everyone in this room, thank you from the bottom of my heart.”

She believed that the Labor Party had failed the electorate.

“The seat of Darling Range we have said over and over in this campaign has been let down,” Ms Hayden said.

“They have been disappointed and they have been abandoned.”

“Today, they sent a strong message to Mark McGowan that I‘ve said a few times this election campaign, my opponent was Mark McGowan.”

Earlier in the morning as the polling booths in Byford opened for the start of voting, Ms Hayden, who currently does not live in the seat, said she would move back to the electorate if the party if she won the seat.

Her previous decision to live outside the electorate had drawn criticism.

However, Opposition Leader Mike Nahan said they had the better candidate.

“We looked around for a candidate and we chose the best,” he said, drawing applause from Liberal Party supporters at the Liberal Party function.

“(She’s) hopeful, experienced, passionate, part of the community.”

“How could we do better?”

He believed with a similar swing in voting “in two and a half years, we will be in government again.”

He said the Labor Party “can’t be trusted” following the exit of former seat holder Barry Urban and candidate Colleen Yates, as well as increasing costs while making cuts to bring the budget back to surplus.

Ms Hayden claimed the people of Darling Range “had spoken on behalf of the entire state of WA” against State Government measures.